An image sensor converts a visual image to digital data that may be represented by a picture. The image sensor comprises an array of pixels, which are unit devices for the conversion of the visual image into the digital data. Digital cameras and optical imaging devices employ an image sensor. Image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While CMOS image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
Referring to FIG. 1, an exemplary prior art device, which is a portion of a CMOS image sensor pixel, includes semiconductor substrate 108 and a gate structure for a transfer gate transistor. The semiconductor substrate 108 comprises a p+ doped semiconductor layer 110, a p− doped semiconductor layer 112, and a shallow trench isolation structure 120. The semiconductor substrate 108 further comprises a photodiode and a surface pinning layer 134 having a p-type doping. The photodiode comprises an n-type charge collection well 130 located beneath the surface pinning layer 134 and a p-type well 132, which is a portion of the p− doped semiconductor layer 112 and vertically abuts the p+ doped semiconductor layer 110. The transfer gate transistor is integrally formed with the photodiode (130, 132) such that the n-type charge collection well 130, which comprises an n+ doped semiconductor material, is also a source of the transfer gate transistor. The transfer gate transistor further comprises a floating drain 140 located in the semiconductor substrate 108, a gate dielectric 150 located directly on a portion of the p− doped semiconductor layer that functions as a channel (corresponding to a straight arrow in FIG. 1), a gate electrode 152, and a gate spacer 154.
A p-n junction and a depletion region is formed between the p-type well 132 and the n-type charge collection well 130. A photon impinging on the photodiode (132, 130) generates an electron-hole pair if the photon interacts with the semiconductor material in the photodiode (132, 130). The energy of the photon that induces electron-hole pair generation depends on the type of the semiconductor material in the semiconductor substrate 108, but the wavelength range of photons for the photogeneration of an electron-hole pair is from about 190 nm to about 1,100 nm for silicon, from about 400 nm to about 1,700 nm for germanium, and from about 800 nm to about 2,600 nm for indium gallium arsenide, respectively.
If the electron-hole pair is generated within the depletion region of the photodiode, which comprises the p-type well 134 and the n-type charge collection well 132, the charge carriers (holes and electrons) drift apart due to the kinetic energy imparted to the charge carriers during the photogeneration process. If a minority carrier (a hole in the n-type charge collection well 130 or an electron in the p-type well 132) enters into the depletion region by drifting, the electric field inherent in the depletion region of the photodiode (132, 130) sweeps the carrier across the p-n junction, which then becomes a majority carrier, i.e., a hole in the p-type well 132 or an electron in the n-type charge collection well 130, upon crossing the p-n junction, and producing a photocurrent if the circuit is closed, or accumulates charges. Particularly, if the carrier is an electron, the carrier accumulated in the n-type charge collection well 132. The amount of charge that accumulates in the n-type charge collection well 132 is nearly linear to the number of incident photons (assuming the photons have the same energy distribution). If the minority carrier recombines with the majority carriers within the photodiode (132, 130) prior to entering the depletion region, the minority carrier is “lost” through recombination and no current or charge accumulation results.
During a read out of the charge from the photodiode (132, 130), electrons in the n-type charge collection well 130 is transferred through the body of the transistor to the floating drain 140 of the transfer transistor. The transfer of the charge needs to be complete to maximize the signal strength from the pixel and to avoid any image lag. If there is a potential barrier between the n-type charge collection well 130 and the channel of the transfer transistor, all of the charge may not be transferred during a read operation or a reset operation.
One of the key technological challenges in currently available CMOS image sensors is degradation of image quality due to dark current and bright points, which reduce charge capacity and reduce dynamic range of a CMOS image sensor. Dark current is the leakage current generated in a photodiode of a CMOS image sensor pixel. The dark current has two components, “ideal dark current” and “defect generated dark current.”
The ideal dark current depends on the doping concentrations, band gap, and the temperature of the photodiode, which is typically reverse biased. The ideal dark current comprises injection-diffusion current due to the injection of thermal electrons and holes having a higher energy than the built-in potential energy to an opposite side of the p-n junction, which becomes a minority carrier diffusion current. The ideal dark current also comprises generation-recombination current due to thermal electron-hole generation or recombination within the depletion region. The two components of the ideal dark current are dependent on applied voltage and temperature. The ideal dark current is a result of inherent limitations of the material properties of the photodiode.
The defect-generated dark current is determined by characteristics of individual defects in the structure of the photodiode. Properties of the material employed in the construction of the photodiode and supporting devices induce electrical charges in the photodiode through various mechanisms including junction leakage of the photodiode, leakage current generated around a shallow trench isolation structure, sub-threshold leakage of transistors connected to the photodiode, drain-induced-barrier-lowering (DIBL) leakage current and gate-induced drain leakage current of such transistors, and other leakages through structural defects and/or limitations of structures of the photodiode itself and surrounding structures. Such dark current reduces the dynamic range of affected CMOS image sensor pixels, and degradation of performance of optical devices employing the CMOS image sensors.
Of particular significance among the various causes of leakage current is the leakage current around a shallow trench isolation structure due to the material properties at the interface between the shallow trench isolation structure and the silicon substrate. Particularly, the point defects within the sidewalls of the silicon substrate that adjoin the shallow trench isolation structure generate surface states that functions as leakage paths for electrical charges. Further, the dopant ions in general, and boron ions in particular, that are introduced into the shallow trench isolation structure during ion implantation steps affect surface passivation of the silicon substrate that abut the shallow trench isolation structure, and generate interface charge states that also function as leakage paths for electrical charges.
Presence of structural defects in a semiconductor material near the interface with a shallow trench isolation structure provides leakage paths that contribute to an increase in the dark current. Also, boron diffusion from the semiconductor material into the shallow trench isolation structure increases dark current by generation of surface states by boron ions present near the interface. Further, boron ions are fast diffusers that diffuse through the photodiode region and effectively reduce the p-n junction depletion region of the photodiode. This results in reduction of charge capacity of the photodiode, and may lead to degradation in image quality from a CMOS image sensor pixel.
In view of the above, there exists a need for a CMOS image sensor structure that provides trapping of defects near a shallow trench isolation structure interface and reduction in boron diffusion into a photodiode to provide enhanced performance of the photodiode in terms of reduced dark current and enhanced charge capacity, and methods of manufacturing the same.